In order to manufacture slabs or blocks of agglomerated or ceramic material it is known to use presses for performing compaction by means of vibro-compression of the mixes of said materials.
In the remainder of the description specific reference will be made to the vibro-compression of slabs without however this being understood as having a limiting meaning
A particular configuration of these presses comprises a support surface on which a tray or a mould filled with mix is placed, a vertically movable structure consisting of an outer bell member and a pressing ram sliding vertically inside it between a raised rest position in which it is separated from the mix to be compacted and a working position in which the ram is lowered until it comes into contact with the top surface of the mix to undergo vibro-compression, which may be lined with a sheet.
The vacuum vibro-compression environment, referred to below as “sealed chamber”, is defined perimetrally by the bell member resting on the support surface of the press, below by the support surface itself and above by the ram. Said sealed chamber is connected to air extraction and vacuum generating means able to form the vacuum inside the chamber itself. A series of vibrating devices for generating a vibratory compaction movement is positioned on the press ram.
After the tray or the mould containing the mix has been transferred onto the support surface of the press, the bell member is lowered to form the sealed chamber, de-aeration of the chamber itself is then activated and at the same time the ram is lowered until it comes into contact with the material to be compacted. At this point the vibrating devices are activated so as to impart a vibratory movement to the ram and, at the same time, the ram is pressed with force against the material. The vacuum generating means which suck the air inside the chamber perform de-aeration of the mix; vacuum vibro-compaction is then carried out in order to compact the layer of mix owing to the compressive force exerted by the ram and the vibration imparted to the ram by the motorized vibrators.
According to the prior art in order to impart to the ram a purely vertical (unidirectional) vibration, therefore without horizontal components which would only prejudice the outcome of the compaction operation and subject the press structures to anomalous mechanical stresses, two sets of vibrators with rotating shafts having an eccentric mass are used, with the vibrators of one set counter-rotating with respect to the vibrators of the other set. In particular, a single vibrating device is used in each set, said device being formed usually by one or more rotating-shaft vibrators arranged in a row with coaxial axes. Each row of vibrators thus contains one or more rotating shafts with eccentric masses depending on the exciting vibration force which is to be obtained and the dimensions of the surface of the mix to be compacted. The rotating shafts are normally operated by electric motors or hydraulic motors.
In order to ensure maximum uniformity and efficiency of the single row of vibrators, they are coaxially connected together; therefore the vibrators of a same row all rotate in the same direction of rotation, but the direction of rotation of the vibrators of one row is opposite to the direction of rotation of the other row and therefore the two rows of vibrators counter-rotate with respect to each other.
Each vibrator is provided with one or more eccentric masses and in each row of vibrators these masses are arranged angularly in the same position. Moreover, when the vibrators are operated, the eccentric masses, owing to the minimum energy principle, are automatically arranged in phase opposition, namely the eccentric masses of the vibrators in one row are arranged angularly offset by 180° with respect to the masses of the vibrators in the other row, so as to nullify the horizontal component of the resultant force. Therefore normally it is not necessary to use a mechanical device for synchronizing the counter-rotation of the two rows of vibrating shafts.
It is clear that this type of configuration may be used in an optimum manner for slabs or blocks or articles of any length, by increasing the length of each vibrating device, namely the number of vibrators for each one of the two rows. It is not so simple to solve the problem of an increase in the width.
In order to obtain correct compaction of the material, the vibrating surface during its vibro-compressive movement must preferably perform a purely translatory vertical movement and must move rigidly without undergoing flexing and deformation in the two transverse and longitudinal vertical planes.
If the planar arrangement of the ram can be easily maintained in a direction of extension of the ram parallel to the axial direction of the vibrating devices (for example in the direction of the length of the article) since, as mentioned above, the number of vibrators can be increased for each row thus maintaining a uniform distribution of the forces when there is a variation in length of the slab, the same does not happen in the transverse direction, for example with an increase in the width of the article.
In fact, in this second case, the vibrating devices can be moved way from each other, but the increase in the interaxial distance between the two rows of vibrators increases the interaxial distance of the forces applied on to the ram and therefore the ram is acted on by forces which are increasingly less uniform and tend to deform it in the transverse vertical direction. This adversely affects compaction and may also impair the planar arrangement which is no longer ensured.
Moreover, the vibrating force needed to cause vibration of a ram which has a greater width and therefore heavier weight results in the need to increase the magnitude of the rotating masses on each shaft, but this conflicts with the limitations applicable to the load acting on the bearings.
By way of example, FIG. 1 shows in schematic form a cross-sectional view of a ram 350 of a press according to the prior art provided with two rows of vibrators 310,320.
FIG. 2 shows instead in schematic form a cross-sectional view of a ram 450 of a press of the prior art modified, namely with the ram which has been widened so as to be able to compact articles of greater width. The ram 450 is provided, as in the previous example, with two rows of vibrators 410,420.
It is evident from the drawing that only two rows of vibrators can only form a limited source of vibrating force. Also, in view of the existing limits for construction of the bearings in relation to the speed of rotation required for compacting the slabs, it is not possible to increase the size of the eccentric masses generating the vibration. Moreover, the lack of uniformity of application of the vibrating forces along the length of the ram is evident.
Therefore, when it is required to compact articles with a width greater than the maximum width permitted by the current configuration of the vibrators, a different configuration of said vibrators must be defined in order to obtain the expected result.
In order to solve the problem of correct compaction, the person skilled in the art, however, does not consider it possible to increase the number of vibrating devices arranged alongside each other in order to increase the force and the uniformity of vibration. In fact, it has been found that in such a press an increase in the number of rows (or vibrating devices) in the sets produces, on the contrary, a reduction in the vibrating movement imparted, down to a value of practically zero. In fact, owing to the minimum energy principle, the eccentric masses of a greater number of rows tend to be arranged so that these vibrating movements generated by the rows are self-cancelling and the resultant vibratory movement is practically zero.